A compressed air preparation device of a vehicle, in particular of a utility vehicle, is generally connected to a compressor that is driven directly by the engine. The compressor can be arranged directly on the engine shaft and therefore can always be driven when the engine is rotating, or can be coupled to the engine via a clutch. In feed phases, the compressor feeds compressed air that first passes through an air dryer, which generally has a dry granulate as the desiccant and can, if appropriate, additionally carry out cleaning. The compressed air that is fed and dried is subsequently stored in a compressed air reservoir and can be output to consumer circuits, which can, for example, be brake control circuits of pneumatic brake systems and/or pneumatic ride level control devices with air bellows. In this context, the individual consumer circuits can also have compressed air reservoirs.
In a feed phase or charging phase, the moisture content of the air dryer is therefore increased; and in order to avoid over-saturation of the desiccant and lower the degree of moisture thereof again, a portion of the dry compressed air that has been stored in the compressed air reservoir is fed back again via the air dryer in regeneration phases, and the compressed air that is therefore permeated with moisture is output, for example, to the surroundings. These regeneration phases generally occur directly subsequent to a feed phase, with the result that a fixed air flow rate or a specific portion of the previously fed air flow rate is always used for the regeneration. The initiation of these regeneration phases can take place in a purely mechanical fashion or else electrically by means of the control device.
The use of the air dryer therefore permits the compressed air system to be protected against damaging moisture, in particular condensation, and, against the moisture freezing, and the corresponding consequent damage.
Since the feed phase is associated with engine fuel consumption, in each case a certain amount of energy or a corresponding portion of the previously input fuel is used in the regeneration phases. Given differing load states, in particular in the case of increased consumption of compressed air and therefore in the case of relatively long feed phases, in each case regeneration phases should be interposed where appropriate, and this is disadvantageous, in particular, in the case of relatively long use phases and feed phases.
EP 1 183 172 B1 describes a compressed air brake system in which, in overrun phases in which the movement energy of the vehicle drives the engine and the connected compressor, the compressed air reservoir is charged beyond the usual setpoint value since this additional feeding of compressed air takes place without additional consumption of fuel. As a result, the feed phases in which the engine drives the compressor while consuming fuel can be shortened to a corresponding degree or avoided, with the result that the overall fuel consumption can be reduced.